Electronic mailboxes reside on email servers and are used to store email messages. Electronic mailboxes are connected to the Internet to enable users to send and receive incoming and outgoing email messages. These mailboxes may also be extended to deliver email to mobile wireless communication devices via wireless networks. In the case of a corporation, electronic mailboxes are typically located on email servers at the corporation. On the other hand, mailboxes for small businesses or individuals are typically located on Internet service provider (ISP) email servers.
Mail user agents (MUAs) are applications which use a technique called polling to relay messages from the email server to the mail program at a user's computer or mobile wireless communications device. A MUA is a program running either on a user's personal computing device (mobile or stationary), or on a shared email relay or polling server that checks for new mail on behalf of a multitude of such users.